Fuel Efficiency When Coasting

Motorist

Is it more economical on fuel to shift into neutral and coast, or is it better to shift up to the highest gear in your car.

foggy_lad

it is illegal to coast because you are not in control! The best thing to do is to change to the gear that best suits the speed you are travelling.

Cookie_Monster

Motorist wrote: »

Is it more economical on fuel to shift into neutral and coast, or is it better to shift up to the highest gear in your car.

in a modern car, no. if in neutral the engine needs to be fed petrol to tick over. if a gear is engaged the motion of the car keep the engine turning and no petrol is allowed into the cylinders saving fuel

Pottler

foggy_lad wrote: »

it is illegal to coast because you are not in control! The best thing to do is to change to the gear that best suits the speed you are travelling.

Really? Go on, Really? Do you get points and fines and stuff? How do they go about prosecuting you, is it based on a Gards hunch? "I clearly saw the accused coasting your honour, I could sense it so I could".

Gumbo

Pottler wrote: »

Really? Go on, Really? Do you get points and fines and stuff? How do they go about prosecuting you, is it based on a Gards hunch? "I clearly saw the accused coasting your honour, I could sense it so I could".

Witty comments doesnt make it legal, its still illegal in this country.

challengemaster

kceire wrote: »

Witty comments doesnt make it legal, its still illegal in this country.

I'd love to see the law on that one

2 stroke

I don't know if it is actualy illegal to coast, but its against the rules of the road. When 1 driver decides to drive by his own set of rules it often leads to disaster.

CiniO

kceire wrote: »

Witty comments doesnt make it legal, its still illegal in this country.

Links please...

CiniO

foggy_lad wrote: »

it is illegal to coast because you are not in control!

Not in control of what?

challengemaster

2 stroke wrote: »

I don't know if it is actualy illegal to coast, but its against the rules of the road. When 1 driver decides to drive by his own set of rules it often leads to disaster.

Coasting isn't mentioned anywhere in the rules of the road. We're all taught it's bad driving practice by driving instructors, but it's nowhere in road traffic act or in the rules of the road, which is, by the way, an interpretation of the road traffic act that is easy to read for your every day person

The purpose of the Rules of the Road is to save lives and prevent injury on our
roads. The rules apply to all road users: drivers, pedestrians, motorcyclists, horse
riders and cyclists.
The rules are written in plain English to encourage good behaviour on the road.
The rules ask us to take personal responsibility for our use of the road.
The Rules of the Road have been designed in an easy to read format. It is your
responsibility to read the rules and know them. At the back of the book is a
Glossary that explains some of the terms used. The rules are also published in
Irish.
The rules comply with and reflect the Road Traffic Law as at 31 December 2009.
Where planned changes in the law are known with certainty a comment is
included in the appropriate section to ensure the content reflects them. It will
be important to check the relevant section of the Road Safety Authority web site
www.rsa.ie for updates.
We can save lives and prevent injury by changing our behaviour. Reading these
rules and applying them will help achieve this goal.
It’s our responsibility.
It’s our choice.

2 stroke

Looks like you have me but I'm certain it used be in the old rules of the road book.

CiniO

2 stroke wrote: »

Looks like you have me but I'm certain it used be in the old rules of the road book.

I have this book (one issued in 1997 AFAIR) but it's at my parents house so I'll be able to check it only in few months when I'll be there.

However even if it was in the book, it could be as a good advice, because it was never in the Law.

WallyGUFC

Cookie_Monster wrote: »

in a modern car, no. if in neutral the engine needs to be fed petrol to tick over. if a gear is engaged the motion of the car keep the engine turning and no petrol is allowed into the cylinders saving fuel

How about a 99 Astra?

wonski

It is not illegal nor legal. I don't think there is a country in the world that would have to put this in their legislations.
Normally coasting isn't dangerous at all, but i would not go downhill through the corner at any speed in neutral. Power at the wheels helps a lot. Also there are emergency situations where you may need to accelerate rather than stop...
It is bad driving practice, but can't see a problem when most people do it only at approach (junctions, parkings etc)
No saving of fuel too, as someone mentioned any car with fuel injection will use close to none petrol when foot is off the gas (even 99 Astra). The difference when coasting wouldn't be that much too.

shedweller

kceire wrote: »

Witty comments doesnt make it legal, its still illegal in this country.

As is going over 120kph on the motorway but i don't see anyone complaining about that.

Cookie_Monster

wonski wrote: »

It is not illegal nor legal.

what is it then?

wonski

Cookie_Monster wrote: »

what is it then?

It is bad practice, as is changing your gear to late, or for example slow progress on road, or driving under the limit. Some posters here fail to see that if something can lead to fail driving test, it does not necesarily makes it illegal.

Cookie_Monster

wonski wrote: »

It is bad practice

that it may be but it still has to be either legal or not...

wonski

Cookie_Monster wrote: »

that it may be but it still has to be either legal or not...

I would say since it is not illegal, then it is legal - that is my logic anyway.
I personally don't expect every single aspect of my life, driving, working etc to be described legal/illegal.
For example: (extreme example)
It is not illegal to wear tshirt when outside temp is 2 degrees, it is legal then, but does legality of it make it less stupid?
Yes, you can walk in one shoe only, it is perfectly legal. It is pointless, and uncomfortable tough...

So coasting is not described legal/illegal, but is kind of pointless too. There is no need to look for it in any book, (Cinio will try anyway). What driving instructors are saying about it is right, and since all current cars have MPI engines, and nice and smooth gear change, there is no need to crawl along downhill in neutral. There are some safety issues with it too.

Neilw

I was in China for a few weeks and noticed coasting was common practice, any car I was in the driver would coast it along. They accelerate to speed, then coast in nutral until the car slows down then repeat. Even busses and coaches were doing it

Gumbo

challengemaster wrote: »

I'd love to see the law on that one

CiniO wrote: »

Links please...

Maybe this is a classic example of how something is drilled into you when learning that you took it as gospel Although i learned to drive in different times to the present system so all theat BS may be out of the system by now.

Also there may be some mix up as to what the term coasting means, ie. just letting the car free roll while still turned on or turning the engine off and rolling down a hill for example.

shedweller wrote: »

As is going over 120kph on the motorway but i don't see anyone complaining about that.

hang around here long enough and you soon will

johnayo

Cookie_Monster wrote: »

in a modern car, no. if in neutral the engine needs to be fed petrol to tick over. if a gear is engaged the motion of the car keep the engine turning and no petrol is allowed into the cylinders saving fuel

If no fuel was allowed into the cylinders then the engine would surely cut out.
This is only relevant on cars that have stop-start systems.

piston

How on earth can it be illegal? There have been cars such as some Rovers and and all Saabs of a certain age that have a bicycle style freewheel mechanism. If coasting was illegal, by definition, these cars would never have been allowed on the roads.

Quazzie

johnayo wrote: »

If no fuel was allowed into the cylinders then the engine would surely cut out.
This is only relevant on cars that have stop-start systems.

The momentum of the car keeps it going, and the car diesels with no fuel being used.

johnayo

Quazzie wrote: »

The momentum of the car keeps it going, and the car diesels with no fuel being used.

Does this mean the engine is not running then.

lkdsl

The original question was about coasting - in modern diesel cars you will use less fuel with the car in an appropriate gear and no pressure on the accelerator. This obviously only works downhill or when you want to slow down. Putting the car in neutral (what is being described above) uses more fuel (on the cars I have tried it on). I've no idea why! It may have something to do with the electrical system re-capturing some of the energy - a technique that is a common fuel saving technique in modern engines.

When I learnt to drive (a long time ago) I was taught not to put the car in neutral so that I had immediate control of acceleration/deceleration if required.

Quazzie

johnayo wrote: »

Does this mean the engine is not running then.

When the car is in motion and you take your foot off the accelerator, the electronics ensure that a small amount of fuel is injected into the engine, to to keep the combustion process going. If the car is moving and in gear, the momentum of the car runs the engine as opposed to the opposite which is normal. The fuel used during this process is minuscule and whilst its technically using some fuel it in negligible.

When a car is coasting it is like the car is standing still in neutral and a lot more fuel is required to keep the car running at 700-800 RPM than is need to ensure combustion.

tuxy

Quazzie wrote: »

When a car is coasting it is like the car is standing still in neutral and a lot more fuel is required to keep the car running at 700-800 RPM than is need to ensure combustion.

That's why you should switch your engine off before going down a big hill. Just watch out for the wheel lock

Quazzie

tuxy wrote: »

That's why you should switch your engine off before going down a big hill. Just watch out for the wheel lock

Close your eyes for extra lulz

piston

I remember my dad having a MK4 Cortina back in the day. For whatever reason, the steering lock didn't work on that car. We lived at the bottom of a steep hill and he used to switch the engine off at the top of the hill and coast down it and down the driveway. When he replaced the car with a newer Cortina, the first time out, he switched the engine of as usual and when he went to turn down the drive, the steering locked and we hit the pillar

Where To

piston wrote: »

I remember my dad having a MK4 Cortina back in the day. For whatever reason, the steering lock didn't work on that car. We lived at the bottom of a steep hill and he used to switch the engine off at the top of the hill and coast down it and down the driveway. When he replaced the car with a newer Cortina, the first time out, he switched the engine of as usual and when he went to turn down the drive, the steering locked and we hit the pillar

Careful now!
A man down the pub told me it was illegal todrive with a broken steering lock :P

Seth Brundle

If you coast, then you gradually slow down. To get back up to speed, you have to accelerate which requires addition fuel consumption. Where is the saving made?

MugMugs

kbannon wrote: »

If you coast, then you gradually slow down. To get back up to speed, you have to accelerate which requires addition fuel consumption. Where is the saving made?

What about downhill?

Where To

Fuel efficiency depends on what kind of driver you are in the first place.

I've seen differences of fifty euro per week from different drivers doing the same journeys in the same vehicle

I make a bit of a game out of it sometimes. When you see traffic stopped up ahead see if you can knock it into neutral at the right time and coast up so that the car just stops behind the traffic without having to use the brakes

I have to admit I coast a bit in general but its usually in traffic and coming up to junctions where you are going slow and its difficult to keep the car moving in a lower gear with out using the accelerator, in this case I would be fairly sure coasting uses less fuel than staying in gear. Generally if the hill is steep enough to turn the car over in gear I will stay in gear.

On another note, pressing the clutch while coasting causes the engine to run at lower revs and thus uses less fuel, however its not good for the clutch and also takes away the relaxing aspect of just sitting back and steering which is associated with coasting.

cjt156

Dorothy Ambitious Scrabble wrote: »

pressing the clutch while coasting causes the engine to run at lower revs and thus uses less fuel,

Nope, that's the whole point of the thread.

Whether you're in neutral, or clutch in, you're coasting and the ECU will meter fuel as if you were stopped and idling. Fuel consumption A.

If you are driving in the correct gear, at a reasonable speed, clutch out, without touching the accelerator, the wheels are driving the engine, ECU is not metering fuel. Fuel consumption B.

A is greater than B. In a modern efficient car B can often equal zero.

dr.fuzzenstein

Cookie_Monster wrote: »

that it may be but it still has to be either legal or not...

As a German I am amazed every day with the almost sick obsession the Irish people have with rules, legal, illegal, the law, procedure, paperwork, enforcement, etc...
Been back to Germany a few times recently and the attitude over there is far more relaxed.
One official even told me "Yeah, there's the rules as written down and then there's reality".
So, if you are fed up with being in a small minded country with an almost clinical pathological obsession and fixation on rules, go to Germany.:p

cjt156

Yeah, those crazy, chilled-out Germans, what'll they think of next?

cjt156

cjt156 wrote: »

Nope, that's the whole point of the thread.

Whether you're in neutral, or clutch in, you're coasting and the ECU will meter fuel as if you were stopped and idling. Fuel consumption A.

If you are driving in the correct gear, at a reasonable speed, clutch out, without touching the accelerator, the wheels are driving the engine, ECU is not metering fuel. Fuel consumption B.

A is greater than B. In a modern efficient car B can often equal zero.

No, I'm not disagreeing with you atall, what I meant is coasting in neutral uses more fuel than coasting in gear with the clutch pressed, look at your revs they are lower with the clutch pressed.

However having the wheels turn the engine over, i.e. be in the right gear and moving fast enough but not pressing the accelerator uses less than both of the above I mentioned.

dr.fuzzenstein

cjt156 wrote: »

Yeah, those crazy, chilled-out Germans, what'll they think of next?

Sausages stuffed with more sausage I hear!;)

dougie-lampkin

Neilw wrote: »

I was in China for a few weeks and noticed coasting was common practice, any car I was in the driver would coast it along. They accelerate to speed, then coast in nutral until the car slows down then repeat. Even busses and coaches were doing it

It's called pulse and glide, when used on hybrids it gives an amazing economy increase. Some eejits with too much time on their hands got a stock Prius to 130 mpg with this technique in the states. It can work on any car, but only if you turn the engine off when you're on the glide cycle in neutral. The Prius just happens to do this automatically.

Quazzie wrote: »

When the car is in motion and you take your foot off the accelerator, the electronics ensure that a small amount of fuel is injected into the engine, to to keep the combustion process going. If the car is moving and in gear, the momentum of the car runs the engine as opposed to the opposite which is normal. The fuel used during this process is minuscule and whilst its technically using some fuel it in negligible.

When a car is coasting it is like the car is standing still in neutral and a lot more fuel is required to keep the car running at 700-800 RPM than is need to ensure combustion.

Most injected cars actually use literally 0 fuel when coasting above idle. With a OBD cable connected to my 12 year old Punto, the injector pulse time is reported as 0ms when on the overrun. I haven't tested any other cars, but I'd imagine anything less than 20 years old would be smart enough to shut off the injector(s) when coasting.

The fuel used to keep it idling is also so miniscule in comparison to say WOT at 3,000 RPM that it's almost zero.

Dorothy Ambitious Scrabble wrote: »

On another note, pressing the clutch while coasting causes the engine to run at lower revs and thus uses less fuel, however its not good for the clutch and also takes away the relaxing aspect of just sitting back and steering which is associated with coasting.

I think the power required to turn over the box's input shaft is immeasurable when compared to the power required to turn over the whole engine. So no, pushing the clutch in won't net better fuel economy. Plus the release bearing and pressure plate springs have a certain lifespan, there's no point stressing them.

I can't think why your RPM would drop with the clutch going in, but if it does that means there's actually more load being put on the engine.

dougie-lampkin

dougie-lampkin wrote: »

I think the power required to turn over the box's input shaft is immeasurable when compared to the power required to turn over the whole engine. So no, pushing the clutch in won't net better fuel economy. Plus the release bearing and pressure plate springs have a certain lifespan, there's no point stressing them.

I can't think why your RPM would drop with the clutch going in, but if it does that means there's actually more load being put on the engine.

See my post above, I agree with the fact that its much better to be in gear.

On the clutch in thing, they even did it on top gear once and showed that coasting in neutral uses more fuel than coasting in gear with the clutch pressed, however I don't do it for the very reasons you give above i.e. stressing parts that have a limited life span.

Quazzie

Dorothy Ambitious Scrabble wrote: »

See my post above, I agree with the fact that its much better to be in gear.

On the clutch in thing, they even did it on top gear once and showed that coasting in neutral uses more fuel than coasting in gear with the clutch pressed, however I don't do it for the very reasons you give above i.e. stressing parts that have a limited life span.

If you've pressed the clutch it makes absolutely zero difference what gear you're in.

LLU

Quazzie wrote: »

The momentum of the car keeps it going, and the car diesels with no fuel being used.

Diesel-ing means that the fuel ignites even though the spark plugs are not sparking, so you do use up whatever residual fuel is available in the system even though the ignition was switched off.

Quazzie

Quazzie wrote: »

If you've pressed the clutch it makes absolutely zero difference what gear you're in.

I know, I'm comparing 3 things.

1) Coasting in neutral with both feet off the pedals

2) Coasting in gear or neutral but keeping the clutch pressed (shown to use less fuel than (1) on top gear).

3) In gear, clutch not pressed and not accelerating. i.e. using cars momentum to turn over the engine and using almost no fuel.

(3) is best, then (2) and finally (1).

yaledo

OP, Yes - Coasting uses a lot less fuel than maintaining a steady speed in gear. 'Engine braking' (in gear with feet off the pedals) uses even less than coasting, but this slows you down a lot more than coasting, so you can't do it for as long. So coasting tends to be more efficient than engine braking (unless you're trying to reduce speed in a hurry)

dougie-lampkin wrote: »

It's called pulse and glide, when used on hybrids it gives an amazing economy increase.
...
It can work on any car, but only if you turn the engine off when you're on the glide cycle in neutral.

Even if you don't turn the engine off, this can still give better efficiency than maintaining a steady speed in gear. It varies from car to car, but the theory is that as you accelerate, your engine is in a much more efficient torque range than when you cruise at steady speed.

There's a good explanation of it at
http://www.metrompg.com/posts/pulse-and-glide.htm

Quazzie

Dorothy Ambitious Scrabble wrote: »

I know, I'm comparing 3 things.

1) Coasting in neutral with both feet off the pedals

2) Coasting in gear or neutral but keeping the clutch pressed (shown to use less fuel than (1) on top gear).

3) In gear, clutch not pressed and not accelerating. i.e. using cars momentum to turn over the engine and using almost no fuel.

(3) is best, then (2) and finally (1).

Have you a link to where it states theres a difference between 1 and 2?

Wossack

piston wrote: »

I remember my dad having a MK4 Cortina back in the day. For whatever reason, the steering lock didn't work on that car. We lived at the bottom of a steep hill and he used to switch the engine off at the top of the hill and coast down it and down the driveway. When he replaced the car with a newer Cortina, the first time out, he switched the engine of as usual and when he went to turn down the drive, the steering locked and we hit the pillar

Top of a deserted multistorey, and a friend reckoned he could get to the bottom on momentum alone. Nuther lad in the car said he couldn't, money on it etc. But he didnt believe he wouldnt use a bit of gas, so ignition had to be off following a rolling start..

Hilarity ensues when the power steering doesnt work, catching him out initially... and then the steering locks.. :pac:

mickdw

Wossack wrote: »

Top of a deserted multistorey, and a friend reckoned he could get to the bottom on momentum alone. Nuther lad in the car said he couldn't, money on it etc. But he didnt believe he wouldnt use a bit of gas, so ignition had to be off following a rolling start..

Hilarity ensues when the power steering doesnt work, catching him out initially... and then the steering locks.. :pac:

Neither lad should ever have a driving licence tbh if they were not aware of the issues that were going to arise there.

galwaytt

MugMugs wrote: »

What about downhill?

....how did you get to the top of the hill, to be in a position to coast down the other side............... ??

Quazzie

Quazzie wrote: »

Have you a link to where it states theres a difference between 1 and 2?

No, I just always remember it from an episode of top gear (or some motoring show) about 6 or 7 years ago. I can remember saying it to people at the time rather than remember the actual tests they did etc. Maybe its not true, I will be looking at the mpg read out in the car this evening and try both ways to see if the readout changes.

Wossack wrote: »

Top of a deserted multistorey, and a friend reckoned he could get to the bottom on momentum alone. Nuther lad in the car said he couldn't, money on it etc. But he didnt believe he wouldnt use a bit of gas, so ignition had to be off following a rolling start..

Hilarity ensues when the power steering doesnt work, catching him out initially... and then the steering locks.. :pac:

Leaving the ignition on would have prevented the steering lock engaging.